The invention relates to a piston head of a piston for an internal combustion engine, in particular for diesel engines, comprising an upper part having a piston crown extending substantially perpendicularly to the longitudinal axis of the piston and a lower part connected with the upper part and having two pin bosses, the mutually facing sides of which are arranged at a mutual distance from one another, wherein the pin bosses each comprise a pin hole extending perpendicularly to the longitudinal axis of the piston and in each case aligned with the other and wherein a cavity is formed in the junction area of the upper part and the lower part, which cavity-adjoins the piston crown on the pin boss side and opens into the area between the pin bosses.
The piston skirt may be connected articulatedly to this piston head as a separate component via a piston pin (two-piece piston or floating skirt piston). The piston skirt may also be connected rigidly with the piston head as a unit.
A piston of this type is used inter alia as a lightweight piston in high performance engines, for example lorry engines, in which pressures ranging approximately from 180 to 220 bar are exerted on the piston crown during operation by combustion gases generated in the combustion chamber of a cylinder upon combustion. In order to withstand the loads arising, the pistons have to be of an appropriately stable construction. On the other hand, it is desirable, in order to reduce the effective acceleration forces, to reduce as far as possible the mass and thus the inertia of the pistons. To fulfil these conflicting requirements, it was previously proposed to produce pistons which are provided with material recesses in the less mechanically stressed areas thereof, in order thereby to effect a weight reduction. In the past, this was achieved in particular by providing a cavity in the area beneath the piston crown, i.e. on the side of the piston crown remote from the combustion chamber in the case of pistons installed in the cylinder. However, this area beneath the piston crown is not readily accessible during creation of the cavity at the time of piston manufacture owing to the position of the pin bosses, in particular owing to the slight mutual spacing of the pin bosses, which makes it difficult or even impossible for the cavity to exhibit a large size.
This problem is revealed for example by FIG. 1 of WO 96/22459: owing to the narrow space between the pin bosses, the area beneath the piston crown cannot readily be accessed by tools for the purpose of creating the cavity.
To solve the problem of poor accessibility of the area beneath the piston crown during production of a cavity, it is therefore proposed in generic U.S. Pat. No. 5,150,517 to construct the piston head in two pieces. According to this prior art, the upper part and lower part of the piston head are prefabricated from two separate components and then joined together by friction welding to form a piston head blank. To form the cavity, appropriate material recesses are formed in both components during individual machining and the components are then joined together by welding in such a way that the two material recesses together form the cavity positioned beneath the piston crown. The assembled piston blank is subsequently finished. In this way, a piston may indeed be obtained which has a relatively large cavity and low weight, but such a piston manufacturing method, in particular the joining together of the two components, is complex, which makes the production of such pistons time-consuming and cost-intensive.
Other composite pistons are known from EP 0 697 513 A1 and DE 38 30 033 C2. These publications also disclose pistons in which the cavity is formed by joining together two piston components provided with corresponding material recesses. The piston components are connected by means of a plurality of mechanical connecting elements, such as clips and screws for example, which on the one hand increases susceptibility of the piston to failure and on the other hand is again cost-intensive and time-consuming.
As an alternative to a multi-piece piston design, another way of simplifying creation of a large cavity beneath the piston crown considered in the prior art involves enlarging the mutual spacing of the pin bosses. Through this measure, relatively large tools may be introduced into the space between the pin bosses for machining the area lying beneath the piston crown. A relatively large space between the pin bosses has the disadvantage, however, that a pin inserted into the pin holes for mounting a connecting rod has to be correspondingly longer and is thus exposed to a greater bending load, which has a negative effect on the service life thereof and may lead to engine damage, should the pin fail.
In contrast, the object of the present invention is to provide a piston head of the above-described type which allows the production of a large cavity beneath the piston crown together with simple manufacture and a long service life.
This object is achieved by a piston head of the aforesaid type in which a recess opening into the cavity and extending in the direction of the longitudinal axis of the piston is formed in each case in at least one, preferably in both, of the mutually facing sides of the pin bosses. By providing at least one, preferably two, recesses in the area of the mutually facing sides of the pin bosses, it is possible, despite a small clearance between the pin bosses, to provide additional space for the introduction of a tool into the area beneath the piston crown to produce a relative large cavity beneath the piston crown. It is thus possible to simplify creation of the cavity, while nonetheless keeping the clearance small between the two mutually facing sides of the pin bosses. In this way, the bending load acting on a pin inserted into the pin holes may be kept small. In addition, selecting a small clearance between the pin bosses has the further advantage that the pin provided for mounting the connecting rod has only a small overall length, which in turn leads to a reduction in the mass in motion and thus in the mass moment of inertia.
In order to make the cavity beneath the piston crown as large as possible, provision may be made for the cavity to undercut the mutually facing sides of the pin bosses at least in part in a direction perpendicular to the longitudinal axis of the piston. In this connection, it may be provided that the cavity undercuts the opening of the recess provided in the respective pin boss at least in part in a direction perpendicular to the longitudinal axis of the piston.
The respective recess or the respective recesses in the pin bosses may be defined by part of a cylindrical, preferably circular-cylindrical, circumferential surface, the longitudinal cylinder axis extending in parallel with the longitudinal axis of the piston and preferably coinciding therewith. Production of the respective recess in the pin bosses in the form of part of a circular-cylindrical circumferential surface is unproblematic from the point of view of manufacture, for example it may be produced during forging of the piston head or by means of a drill or cylindrical milling cutter.
With regard to the orientation of the pin bosses relative to one another, provision may be made for the mutual distance between the mutually facing sides of the pin bosses to increase in the direction away from the piston crown, preferably to increase continuously. In the case of such a widening of the space between the pins in the direction away from the piston crown, it may be sufficient, for the purpose of enlarging the access to the area in the vicinity of the piston crown, to provide the recess or the recesses only in the narrowest area of the pin bosses, i.e. in the area of the pin bosses in the vicinity of the upper part, irrespective of whether the distance between the mutually facing sides of the pin bosses increases linearly or non-linearly, i.e. substantially in stages, in the direction away from the piston crown. In the first instance, the pin bosses are trapezoidal. In the event of a non-linear increase in the distance between the pin bosses, on the other hand, the pin bosses are stepped. It should be noted that, where the pin boss spacing is substantially constant in the direction away from the piston crown, the respective recess extends over the entire pin boss height.
In order to ensure reliable, low-play mounting of the connecting rod, i.e. of the connecting rod small end, between the pin bosses in the case of a piston installed in an engine, provision may be made for the mutually facing sides of the pin bosses to comprise mutually opposing bearing faces disposed orthogonally in relation to the pin hole axis (top-guided connecting rod). When the piston is installed, these bearing faces come to bear against corresponding bearing faces of the connecting rod small end and stabilize the swiveling motion of the connecting rod against undesirable tilting out of the swivel plane. Where the space between the pin bosses widens in the direction away from the piston crown, such bearing faces are provided only in the area of the pin bosses in the vicinity of the upper part.
With regard to the arrangement of the respective recess relative to the bearing faces on the pin bosses, provision may be made for the bearing face to be formed in each case of a pair of bearing faces lying a common plane, between which pair the respective recess is provided. By positioning the respective recess in such a way between the bearing faces of the bearing face pair, stable mounting of the connecting rod small end between the pin bosses is still ensured despite the reduction in size of the overall bearing face on each pin boss, which reduction in size arises because the bearing face portion is omitted in the area of the recess, in contrast to a pin boss without the recess according to the invention. In this context, it should also be noted that, owing to the possibility of providing a small distance between the pin bosses, the bending forces acting on the pin mounting the connecting rod small end may be kept slight, such that the pin bends only minimally in the operating state and thus undesirable tilting of the connecting rod out of its swivel plane is minimized.
In order to take account of the above-described operating conditions, i.e. the relatively high operating pressures, provision may be made for the piston head to be made of a high strength material, preferably of steel. However, it is in principle also possible to make the piston head of aluminum material or aluminum alloys if the piston head according to the invention is to be used under lower operating pressures, for example in the region of 150 bar.
The invention further relates to a method of producing a piston head, in particular a piston head of the above-described type, comprising the stages
(A) production of a piston head blank, comprising an upper part having a piston crown extending substantially perpendicularly to the longitudinal axis of the piston and additionally comprising a lower part connected with the upper part and having two mutually spaced pin bosses;
(B) formation of a recess in at least one of the mutually facing sides of the pin bosses, in each case in the direction of the longitudinal axis of the piston; and
(C) formation of a cavity in the junction area of upper part and lower part by means of a tool passed through between the pin bosses in the area of the recess, of which there is at least one.
Provision may be made according to the invention for stages (A) and (B) to comprise forging processes, resulting in wear-resistant pistons. It may additionally be provided that stage (C) comprises cutting machining, which allows simple, low-cost production of a cavity beneath the piston crown.